Transcript unit 2 - dispersion of pollutants

UNIT 2 - DISPERSION OF POLLUTANTS
2.1. Elements of atmosphere
2.2. Meteorological factors
2.3. Wind roses
2.4. Lapse rate
2.5. Atmospheric stability and turbulence
2.6. Plume rise
2.7. Dispersion of pollutants
2.8. Dispersion models
2.9. Applications.
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2.1. Elements of atmosphere
 Earth’s atmosphere is a layer of gases
surrounding the planet.
 The Earth is surrounded by a blanket
of air, which we call the atmosphere.
It reaches over 560 kilometers from
the surface of the Earth.
Atmosphere:
 Absorbs the energy from the Sun,
 Recycles water and other chemicals,
 protects us from high-energy radiation
and the frigid vacuum of space.
 The atmosphere protects and supports
life.
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2.1. Elements of atmosphere
contd…
 Earth’s atmosphere is made of a
mixture of gases called air.
 Nitrogen gas makes up about 78% of
Earth’s atmosphere.
 The second most abundant gas is
oxygen, which makes up 21% of
Earth’s atmosphere.
 The third Argon (Ar, 0.9%).
 Carbon Dioxide (CO2, 0.03%).
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2.1. Elements of atmosphere
contd…
Composition of the Atmosphere
The atmosphere is comprised of a variety of gases:
 Major Constituents (99%):
 Nitrogen (N): 78%
 Oxygen (O2): 21%
 Trace Constituents:
 Argon (Ar), about 0.9%
 Water vapor (H2O), up to 10000 ppmv
 Carbon dioxide (CO2), 350 ppmv
 Ozone (O3), near zero at the surface, up to 10 ppmv in the
stratosphere
 Methane (CH4), 1.7 ppmv
 and others…..
ppmv = “parts per million by volume”
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2.1. Elements of atmosphere
Nitrogen Cycle
contd…
 Nitrogen is important to
protein which is found in
the body tissues of all
living things.
 Nitrogen is cycled through
the soil and into plants and
finally when living things
die and decay.
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2.1. Elements of atmosphere
Pressure in the atmosphere
contd…
 Atmospheric pressure is the
force per unit area exerted
into a surface by the weight
of air above that surface in
the atmosphere of Earth.
 The gas molecules closest to
Earth’s surface are packed
together very closely.
 This means pressure is
lower the higher up you go
into the atmosphere.
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2.1. Elements of atmosphere
contd…
Pressure in the atmosphere
 At sea level, the weight of the
column of air above a person is
about 9,800 Newtons (2,200
pounds)!
 This is equal to the weight of a small
car.
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2.1. Elements of atmosphere
contd…
Pressure changes with altitude
Pressure varies smoothly from
the Earth's surface to the top of
the mesosphere.
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2.1. Elements of atmosphere
contd…
Measuring Pressure
 A barometer is an instrument that
measures atmospheric pressure.
 Long ago, mercury barometers
were used
 Since mercury is a poisonous
liquid, aneroid barometers are
used today.
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2.1. Elements of atmosphere
Layers of Atmosphere
contd…
The atmosphere has four layers
Thermosphere
Mesosphere
Stratosphere
Troposphere
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2.1. Elements of atmosphere
Layers of Atmosphere
contd…
Troposphere
 Lowest and thinnest layer
 16 km at equator, 8 km at poles
 90% of the atmosphere’s mass
 Temperature decreases with altitude
 6°C per kilometer
 Top of troposphere averages –50°C
View of troposphere layer from an
airplane's window.
 Where weather occurs
 Boundary between the troposphere, and the stratosphere is called
the tropopause
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2.1. Elements of atmosphere
contd…
Layers of Atmosphere
Stratosphere
 Extends from 10 km to 50 km above the ground
 Less dense (less water vapor)
 Temperature increases with altitude
 Almost no weather occurrence
 Contains high level of ozone
 Ozone layer
 Upper boundary is called
stratopause.
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2.1. Elements of atmosphere
contd…
Layers of Atmosphere
Mesosphere
 Extends to almost 80 km high
 Gases are less dense.
 Temperature decreases as altitude increases.
 Gases in this layer absorb very little UV radiation.
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2.1. Elements of atmosphere
contd…
Layers of Atmosphere
Thermosphere
 Above the mesosphere and extends to
almost 600 km high
 Temperature increases with altitude
 Readily absorbs solar radiation
 Temperature can go as high as 1,500 °C
 Reflects radio waves
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2.1. Elements of atmosphere
contd…
Layers of
Atmosphere
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2.1. Elements of atmosphere
contd…
Layers of the
Atmosphere
The four layers of the
atmosphere include:
1. the troposphere, where we
live;
2. the stratosphere, which
contains the ozone layer;
3. the mesosphere, where
meteors burn; and
4. the thermosphere, where
satellites orbit Earth.
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2.1. Elements of atmosphere
contd…
Layers of the Atmosphere
 The exosphere begins at about
500 kilometers above Earth
and does not have a specific
outer limit.
 Satellites orbit Earth in the
exosphere.
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2.1. Elements of atmosphere
contd…
The exosphere and ionosphere
 Communication on Earth
depends on satellites.
 Satellites transmit
information used for
television shows, radio
broadcasts, data and photos
used in weather reports,
and long distance telephone
calls.
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The ozone layer
 In the 1970s, scientists
noticed that the ozone
layer in the stratosphere
above
Antarctica
was
thinning.
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Chlorofluorocarbons & the ozone layer
 A group of chemicals called
chlorofluorocarbons (or CFCs)
were once commonly used in air
conditioners, in aerosol spray
cans, and for cleaning machine
parts.
 In the London Agreement of
1991, more than 90 countries
banned the production and use
of CFCs except for limited
medical uses.
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Chlorofluorocarbons & the ozone layer
 The ozone layer absorbs the Sun’s high-energy ultraviolet (UV)
radiation and protects the Earth.
 In the stratosphere, the CFCs break down and release chlorine.
 The chlorine reacts with ozone molecules, which normally block
incoming ultraviolet radiation.
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Acid rain occurs when oxides of sulfur and oxides
of nitrogen are emitted into the atmosphere,
undergo chemical transformations and are
absorbed by water droplets in clouds.
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Effects of Acid Rain
Acidification of bodies of water
Damage of vegetation
Damage to building materials, statues, etc.
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2.2. Meteorological factors
• Air movements influence the fate of air pollutants.
• So any study of air pollution should include a study of
the local weather patterns (meteorology).
• If the air is calm and pollutants cannot disperse, then
the concentration of these pollutants will build up.
• On the other hand, when strong, turbulent winds blow,
pollutants disperse quickly, resulting in lower pollutant
concentrations.
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2.2. Meteorological factors
contd…
Meteorological data helps:
1. Identify the source of pollutants
2. Predict air pollution events such as
inversions and high-pollutant concentration
days
3. Simulate and predict air quality using
computer models.
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2.2. Meteorological factors
contd…
• When studying air quality, it is important to
measure the following factors as they can help
understand the chemical reactions that occur
in the atmosphere:
– wind speed and direction
– temperature
– humidity
– rainfall
– solar radiation.
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2.2. Meteorological factors
contd…
• Primary Metrological Parameter
– Wind speed, Wind Direction, Atmospheric
Stability
• Secondary Metrological Parameter
– Sunlight
– Temperature
– Precipitation and humidity
– Topography
– Energy from the sun and earth’s rotation drives
atmospheric circulation
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2.2. Meteorological factors
contd…
1. Wind speed and direction
• When high pollutant concentrations occur at a
monitoring station, wind data records can
determine the general direction and area of the
emissions.
• Identifying the sources means planning to reduce
the impacts on air quality can take place.
• An instrument called an anemometer measures
wind speed. At our monitoring stations, the type
of anemometer we use is a sonic anemometer.
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2.2. Meteorological factors
contd…
• A sonic anemometer operates on the principle
that the speed of wind affects the time it takes
for sound to travel from one point to another.
• Sound travelling with the wind will take less time
than sound travelling into the wind.
• By measuring sound wave speeds in 2 different
directions at the same time, sonic anemometers
can measure both wind speed and direction.
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2.2. Meteorological factors
•
contd…
2.
Temperature
Measuring temperature supports air quality assessment, air
quality modeling and forecasting activities.
• Temperature and sunlight (solar radiation) play an important
role in the chemical reactions that occur in the atmosphere to
form photochemical smog from other pollutants.
• Favorable conditions can lead to increased concentrations of
smog.
• The most common way of measuring temperature is to use a
material with a resistance that changes with temperature,
such as platinum wire. A sensor measures this change and
converts it into a temperature reading.
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2.2. Meteorological factors
3. Humidity
contd…
• Like temperature and solar radiation, water vapour plays an
important role in many thermal and photochemical
reactions in the atmosphere.
• As water molecules are small and highly polar, they can
bind strongly to many substances.
• If attached to particles suspended in the air they can
significantly increase the amount of light scattered by the
particles (monitoring aerosols).
• If the water molecules attach to corrosive gases, such as
sulfur dioxide, the gas will dissolve in the water and form
an acid solution that can damage health and property.
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2.2. Meteorological factors
contd…
• Reporting of the water vapour content of air is as a percentage of
the saturation vapour pressure of water at a given temperature.
• This is the relative humidity. The amount of water vapour in the
atmosphere is highly variable—it depends on geographic location,
how close water bodies are, wind direction and air temperature.
•
Relative humidity is generally higher during summer when
temperature and rainfall are also at their highest.
• Measuring humidity uses the absorption properties of a polymer
film.
• The film either absorbs or loses water vapour as the relative
humidity of the ambient air changes.
• A sensor measures these changes and converts them into a
humidity reading.
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2.2. Meteorological factors
4. Rainfall
contd…
• Rain has a 'scavenging' effect when it washes
particulate matter out of the atmosphere and
dissolves gaseous pollutants.
• Removing particles improves visibility. Where
there is frequent high rainfall, air quality is
generally better.
• If the rain dissolves gaseous pollutants, such as
sulfur dioxide, it can form acid rain resulting in
potential damage to materials or vegetation.
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2.2. Meteorological factors
contd…
• A common method to measure rainfall is to
use a tipping bucket rain gauge—see
illustration.
• The gauge registers rainfall by counting
small amounts of rain collected.
• When rain falls into the funnel, it runs into
a container (the tipping bucket) divided
into 2 equal compartments by a partition.
• The design of the tipping bucket makes one
compartment tilt downward and rest
against a stop when it is empty, positioning
the other compartment under the funnel
ready to receive rain water.
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2.2. Meteorological factors
contd…
• When a set amount of rain has drained from the funnel
into the upper compartment the bucket tilts the
opposite way so that the compartment containing the
rain comes to rest against the stop on the opposite
side.
• The collected water then empties out and the other
compartment starts to fill.
• The instrument calculates the quantity and intensity of
rainfall using with the area of the funnel and the
number and rate of bucket movements.
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2.2. Meteorological factors
contd…
5. Solar radiation
• It is important to monitor solar radiation for use
in modeling photochemical smog events, as the
intensity of sunlight has an important influence
on the rate of the chemical reactions that
produce the smog.
• The cloudiness of the sky, time of day and
geographic location all affect sunlight intensity.
• An instrument called a pyranometer measures
solar radiation from the output of a type of
silicon cell sensor.
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2.3. Wind rose
• A wind rose is a graphic tool used by meteorologists to give
a succinct view of how wind speed and direction are
typically distributed at a particular location.
• Historically, wind roses were predecessors of the compass
rose (found on maps), as there was no differentiation
between a cardinal direction and the wind which blew from
such a direction.
• Using a polar coordinate system of gridding, the frequency
of winds over a long time period is plotted by wind
direction, with color bands showing wind ranges.
• The directions of the rose with the longest spoke show the
wind direction with the greatest frequency.
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2.3. wind rose
contd…
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2.3. wind rose
contd…
Uses
• Presented in a circular format, the modern wind rose
shows the frequency of winds blowing from particular
directions over a specified period.
• The length of each "spoke" around the circle is related
to the frequency that the wind blows from a particular
direction per unit time.
• Each concentric circle represents a different frequency,
emanating from zero at the center to increasing
frequencies at the outer circles.
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2.3. wind rose
contd…
• A wind rose plot may contain additional information, in that
each spoke is broken down into color-coded bands that
show wind speed ranges.
• Wind roses typically use 16 cardinal directions, such as
north (N), NNE, NE, etc., although they may be subdivided
into as many as 32 directions.
• In terms of angle measurement in degrees, North
corresponds to 0°/360°, East to 90°, South to 180° and West
to 270°.
• Compiling a wind rose is one of the preliminary steps taken
in constructing airport runways, as aircraft typically
perform their best take-offs and landings pointing into the
wind.
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2.4. lapse rate
• As a parcel of air rises in the earth's atmosphere
it experiences lower and lower pressure from the
surrounding air molecules, and thus it expands.
• This expansion lowers its temperature. Ideally, if
it does not absorb heat from its surroundings and
it does not contain any moisture, it cools at a rate
of 1ºC/100 m rise.
• This is known as dry adiabatic lapse rate. If the
parcel moves down it warms up at the same rate.
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2.4. lapse rate
contd…
• For a particular place at a particular time, the
existing temperature can be determined by
sending up a balloon equipped with a
thermometer.
• The balloon moves through the air, and not with
it.
• The temperature profile of the air, which the
balloon measures, is called the ambient lapse
rate, environmental lapse rate, or the prevailing
lapse rate.
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2.4. lapse rate
contd…
• A super-adiabatic lapse rate also called a
strong lapse rate occurs when the atmosphere
temperature drops more than 1oC/100m.
• A sub-adiabatic rate also called weak lapse
rate, is characterized by drop of less than
1oC/100 m.
• A special case of weak lapse rate is the
inversion, a condition which has warmer layer
above colder air.
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2.4. lapse rate
contd…
• During
super-adiabatic
lapse
rate
atmospheric conditions are unstable.
the
• This is illustrated in Figure. If a parcel of air at
500m elevation, at 20oC is pushed upward to
1000m, its temperature will come down to 15oC
(according to adiabatic lapse rate).
• The prevailing temperature is however 10oC at
1000m. The parcel of air will be surrounded by
colder air and therefore will keep moving up
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2.4. lapse rate
contd…
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2.4. lapse rate
contd…
• The parcel of air will be surrounded by colder
air and therefore will keep moving up.
• Similarly if the parcel is displaced downwards,
it will become colder than its surroundings
and therefore will move down.
• Super adiabatic conditions are thus unstable,
characterized by a great deal of vertical air
movement and turbulence.
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2.4. lapse rate
contd…
• The sub-adiabatic condition shown in Figure (b) is by
contrast a very stable system.
• Consider again a parcel of air at 500 m elevation at
20oC. If the parcel is displaced to 1000 m it will cool by
5oC to 15oC. But the surrounding air would be warmer.
• It will therefore fall back to its point of origin. Similarly
if a parcel of air at 500 m is pushed down, it will
become warmer than its surrounding and therefore
will rise back to its original position.
• Thus such systems are characterized by very limited
vertical mixing
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2.4. lapse rate
contd…
Inversion
• An inversion is an extreme sub-adiabatic condition, and
thus the vertical air movement within the inversion is
almost nil.
• The two most common kind of inversion are subsidence
inversion and radiation inversion. These are illustrated in
Figure .
• The base of the subsidence inversion lies some distance
above earth's surface.
• This type of inversion is formed due to adiabatic
compression and warming of sinking air mass to a lower
altitude in the region of a high pressure center.
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2.4. lapse rate
contd…
• In the case of radiation inversion, the surface layers of the
atmosphere during the day receive heat by conduction,
convection and radiation from the earth's surface and are
warmed.
• This results in a temperature profile in the lower atmosphere,
which is represented by a negative temperature gradient.
• On a clear night, the ground surface radiates heat and quickly
cools.
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2.5. Atmospheric stability and turbulence
• The ability of the atmosphere to enhance or to resist
atmospheric motions
• Influences the vertical movement of air.
• If the air parcels tend to sink back to their initial level after
the lifting exerted on them stops, the atmosphere is stable.
• If the air parcels tend to rise vertically on their own, even
when the lifting exerted on them stops, the atmosphere is
unstable.
• If the air parcels tend to remain where they are after lifting
stops, the atmosphere is neutral.
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2.5. Atmospheric stability and turbulence
contd….
• The stability depends on the ratio of
suppression to generation of turbulence.
• The stability at any given time will depend
upon static stability ( related to change in
temperature with height ), thermal turbulence
( caused by solar heating ), and mechanical
turbulence (a function of wind speed and
surface roughness).
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2.5. Atmospheric stability and turbulence
contd….
Stability classified into 6 classes (A – F)
A: strongly unstable
B: moderately unstable
C: slightly unstable
D: neutral
E: slightly stable
F: moderately stable
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2.5. Atmospheric stability and turbulence
contd….
• Atmospheric stability can be determined using
adiabatic lapse rate.
Γ > Γd
Unstable
Γ = Γd
Neutral
Γ < Γd
Stable

Γ is environmental lapse rate

Γd is dry adiabatic lapse rate (10c/100m) and dT/dZ = -10c /100 m
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2.5. Atmospheric stability and turbulence
contd….
Atmospheric Stability Classification
• Schemes to define atmospheric stability are:
–
–
–
–
–
–
–
–
–
–
–
P- G Method
P-G / NWS Method
The STAR Method
BNL Scheme
Sigma Phi Method
Sigma Omega Method
Modified Sigma Theta Method
NRC Temperature Difference Method
Wind Speed ratio (UR) Method
Radiation Index Method
AERMOD Method (Stable and Convective cases)
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2.5. Atmospheric stability and turbulence
contd….
Turbulence
• Fluctuations in wind flow which have a
frequency of more than 2 cycles/ hr
• Types of Turbulence
–
–
–
–
Mechanical Turbulence
Convective Turbulence
Clear Air Turbulence
Wake Turbulence
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2.6. Plume rise
• Pollutants emitted in plume form
Impact on air quality depends on dispersion,
which depends on the height of plume
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2.6. Plume rise
contd…
• Plume rise affects transport
– Effects maximum ground level concentrations (MGLCs)
– Effects distance of MGLCs
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2.6. Plume rise
contd…
Types of Air Quality Models
 Types of air quality models
 Emission rate Modeling
 Ambient Air Concentration Modeling
 Types of ground level concentration models
 Physical Model / Mathematical Model
 Historical Model
 Trend Model
 Prototype Model
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2.6. Plume rise
contd…
Basic Segments of an Elevated Plume
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2.6. Plume rise
contd…
Basic Segments of An Elevated Plume
Initial phase
• Vertical Jet : Effluents are not deflected immediately upon entering the cross flow if
(Vs / U > 4 )
• Bent-Over Jet Section : Entrainment of the cross flow is rapid because by this time
appreciable growth of vortices has taken place.
• Thermal Section : Self generated turbulence causes mixing and determines the
growth of plume.
Transition phase
• Plume's internal turbulence levels have dropped enough so that the atmospheric
eddies in the inertial sub range determines the plume's growth.
Diffusion phase
• The plume's own turbulence has dropped and energy containing eddies of
atmospheric turbulence determine the growth of plume
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2.6. Plume rise
contd…
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2.6. Plume rise
contd…
Dispersion of Heavy Gases
• Initial Acceleration Phase
• Initial Dilution Phase
• Slumping Phase (internal buoyancydominated dispersion )
• Transition Phase

Passive Phase ( atmospheric turbulence-dominated
dispersion )
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2.6. Plume rise
contd…
• Types of Plume
– Continuous Plume: The release and the sampling
time are long compared with the travel time.
– Puff Diffusion / Instantaneous Plume: The release
time or sampling time is short when compared with
the travel time
• Types of Plume Rise
– Buoyancy Effect: Rise due to the temperature
difference between stack plume and ambient air.
• Momentum Rise: Rise due to exit velocity of the
effluents (emissions).
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2.6. Plume rise
contd…
Concept of Plume Penetration
• Meteorology plays an important role in the
dispersion of effluents.
• Various meteorological factors affect the
dispersion of emission into the atmosphere
in a variety of ways.
• Convective boundary layer (or mixing height)
is one of the most important meteorological
variables responsible for high ground level
concentrations.
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2.6. Plume rise
contd…
Concept of Plume Penetration
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2.6. Plume rise
contd…
Effect of Temperature Profile on Plume Rise
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2.6. Plume rise
contd…
Stack Plume: Looping
•High degree of convective
turbulence
•Superadiabatic lapse rate -- strong
instabilities
•Associated with clear daytime
conditions accompanied by strong
solar heating & light winds
•High probability of high
concentrations sporadically at
ground level close to stack.
•Occurs in unstable atmospheric
conditions.
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2.6. Plume rise
contd…
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2.6. Plume rise
contd…
Stack Plume: Coning
•Strong wind, no turbulence
•Stable with small-scale turbulence
•Associated with overcast moderate to strong winds
•Roughly 10° cone
•Pollutants travel fairly long distances before reaching ground level in significant
amounts
•Occurs in neutral atmospheric conditions
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2.6. Plume rise
contd…
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2.6. Plume rise
contd…
Stack Plume: Fanning
• Occurs under large negative
lapse rate
• Strong inversion at a
considerable distance above
the stack
• Extremely stable atmosphere
• Little turbulence
• If plume density is similar to
air, travels downwind at
approximately same elevation
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2.6. Plume rise
contd…
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2.6. Plume rise
contd…
Stack Plume: Fumigation
• Most dangerous plume: contaminants are all coming down to ground
level.
• They are created when atmospheric conditions are stable above the
plume and unstable below.
• This happens most often after the daylight sun has warmed the
atmosphere, which turns a night time fanning plume into fumigation for
about a half an hour.
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2.6. Plume rise
contd…
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2.6. Plume rise
contd…
Stack Plume: Lofting
• Favorable in the sense that fewer impacts at ground level.
• Pollutants go up into environment.
• They are created when atmospheric conditions are unstable above the
plume and stable below.
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2.6. Plume rise
contd…
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2.6. Plume rise
contd…
Stack Plume: Trapping
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2.6. Plume rise
Effect of Surface Discontinuity
contd…
(a)
Warm land
(b)
Warm land
(c)
Cold land
Cold water
Cold water
Warm water
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2.6. Plume rise
contd…
Effects of Terrain on the Plume Pattern
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2.6. Plume rise
contd…
Impact of Building and Stack Location
Backwash
Downwash
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2.6. Plume rise
contd…
Impact of Stack Height: Stack Upwind of Building
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2.6. Plume rise
contd…
Impact of Stack Height: Building Supported Stack
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2.6. Plume rise
contd…
Impact of Stack Height: Stack Downwind of Building
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2.6. Plume rise
contd…
Cases of Downwash
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2.6. Plume rise
contd…
Plume Affected by Natural Terrain Irregularity
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2.6. Plume rise
contd…
Plume Near Very Large Obstacle
Unstable
Stable
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2.6. Plume rise
contd…
Plume in a Valley
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2.6. Plume rise
contd…
Heat Island Effect
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2.6. Plume rise
contd…
Plume Affected by Heat Island Effect
Toward a city
Within a city
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2.7. Air pollution dispersion
• Air pollution dispersion distribution of air pollution into the
atmosphere.
• Air pollution is the introduction of particulates, biological
molecules, or other harmful materials into Earth's
atmosphere, causing disease, death to humans, damage to
other living organisms such as food crops, or the natural or
built environment.
• Air pollution may come from anthropogenic or natural
sources. Dispersion refers to what happens to the pollution
during and after its introduction; understanding this may
help in identifying and controlling it.
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2.7. Air pollution dispersion
contd…
• Air pollution dispersion has become the focus
of environmental conservationists and
governmental
environmental
protection
agencies (local, state, province and national)
of many countries (which have adopted and
used much of the terminology of this field in
their laws and regulations) regarding air
pollution control.
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2.8. Dispersion Modeling
Dispersion is the process of spreading out the emission
over a large area thereby reducing the concentration of
the pollutants.
Plume dispersion is in two dimensions:
horizontal and vertical.
It is assumed that the greatest concentration of the
pollutants is on the plume centerline in the direction
of the prevailing wind.
The further the away from the centerline the lower
the concentration.
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2.8. Dispersion Modeling
contd…
Air Quality Modeling (AQM)
• Predict pollutant concentrations at various locations
around the source.
• Identify source contribution to air quality problems.
• Assess source impacts and design control strategies.
• Predict future pollutant concentrations from
sources after implementation of new regulatory
programs.
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2.8. Dispersion Modeling
contd…
Areas Surrounding the Site of Release
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2.8. Dispersion Modeling
contd…
Air Quality Modeling (AQM)
• Mathematical and numerical techniques are used in
AQM to simulate the dispersion of air pollutants.
• Modeling of the dispersion of pollutants
– Toxic and odorous substances
– Single or multiple points
– Point, Area, or Volume sources
• Input data required for Air Quality Modeling
– Source characteristics
– Meteorological conditions
– Site and surrounding conditions
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2.8. Dispersion Modeling
contd…
Ambient Air Concentration Modeling
Types of Pollutant Sources
Point Sources
• e.g., stacks or vents
Area Sources
• e.g., landfills, ponds, storage piles
Volume Sources
• e.g., conveyors, structures with multiple vents
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2.8. Dispersion Modeling
contd…
Air Quality Models
STATISTICAL
DETERMINISTIC
PHYSICAL
REGRESSION
STEADY STATE
EMPIRICAL
WINDTUNNEL
SIMULATION
TIME DEPENDENT
GAUSSIAN PLUME
BOX
GRID
SPECTRAL
EULERIAN
PUFF
TRAJECTORY
LAGRANGIAN
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2.8. Dispersion Modeling
contd…
Factors Affecting Dispersion of Pollutants In The Atmosphere
 Source Characteristics
Emission rate of pollutant
Stack height
Exit velocity of the gas
Exit temperature of the gas
–
Stack diameter
Meteorological Conditions
Wind velocity
Wind direction
Ambient temperature
Atmospheric stability
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2.8. Dispersion Modeling
contd…
• There are four types of air pollution dispersion models, as well as
some hybrids of the five types:
1. Box model — The box model is the simplest of the model types.
• It assumes the airshed (i.e., a given volume of atmospheric air in a
geographical region) is in the shape of a box.
• It also assumes that the air pollutants inside the box are
homogeneously distributed and uses that assumption to estimate
the average pollutant concentrations anywhere within the airshed.
• Although useful, this model is very limited in its ability to accurately
predict dispersion of air pollutants over an airshed because the
assumption of homogeneous pollutant distribution is much too
simple.
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2.8. Dispersion Modeling
contd…
2. Gaussian model — The Gaussian model is perhaps the oldest (circa
1936) and perhaps the most commonly used model type.
• It assumes that the air pollutant dispersion has a Gaussian
distribution, meaning that the pollutant distribution has a normal
probability distribution.
•
Gaussian models are most often used for predicting the dispersion
of continuous, buoyant air pollution plumes originating from
ground-level or elevated sources.
• Gaussian models may also be used for predicting the dispersion of
non-continuous air pollution plumes (called puff models).
• The primary algorithm used in Gaussian modeling is the Generalized
Dispersion Equation For A Continuous Point-Source Plume
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2.8. Dispersion Modeling
contd…
Gaussian Models
 Advantages
Produce
results that match closely with experimental
data
Incorporate
Simple
in their mathematics
Quicker
Do
turbulence in an ad-hoc manner
than numerical models
not require super computers
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2.8. Dispersion Modeling
contd…
Gaussian Models
 Disadvantages
•
Not suitable if the pollutant is reactive in nature
•
Fails to incorporate turbulence in comprehensive sense
•
Unable to predict concentrations beyond radius of
approximately 20 Km
•
For greater distances, wind variations, mixing depths and
temporal variations become predominant
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2.8. Dispersion Modeling
contd…
Sources of error in gaussian model
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2.8. Dispersion Modeling
contd…
The spread of the plume is approximated by Gaussian
probability curve
C(x,y,z) = [Q/(2 B u Fy Fz)] exp[ -1/2 [(y / Fy)2 + (z / Fz)2]]
C(x, y, z) = concentration at some point in coordinate space,
kg,m3
Q = source strength, kg/sec
Fy,Fz = standard deviation of the dispersion in the y and z
directions
y = distance crosswind horizontally, m
z = distance crosswind vertically, m
z is in the vertical direction, y is in the horizontal crosswind direction,
and x is the downwind direction
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2.8. Dispersion Modeling
contd…
C(x,y,z) = [Q/(2 B u Fy Fz)] exp[ -1/2 [(y / Fy)2 + (z / Fz)2]]
The degree of dispersion is controlled by the standard deviations
in the equation. When F is large the spread is great, so the
concentration is low (the mass is spread out over a larger area.)
Dispersion is dependent on both atmospheric stability and distance
from the source
The values for the standard deviations for this equation can be
found using tables which have been prepared for that purpose.
To use the table you first must estimate the atmospheric stability
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2.8. Dispersion Modeling
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2.8. Dispersion Modeling
contd…
Consider this figure:
•
A plume emitted from a stack
has an effective height H (you
have to calculate h).
The
centerline of the plume, z, is
then H and the dispersion
equation becomes:
Soln :
C(x,y,z) = [Q/(2 B u Fy Fz)] exp[ -1/2 [(y / Fy)2 + (z - H / Fz)2]]
This equation and the one presented previously hold as long as the
ground does not influence the diffusion (the plume hits the ground)
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2.8. Dispersion Modeling
contd…
• Since most pollutants are not absorbed by the ground, and they
can not diffuse into the ground the equations above will not
work when there is influence from the ground.
• One way of accounting for this influence is to assume all
pollutants are reflected by the ground.
A new equation can be written based on
this assumption:
C(x,y,z) = [Q/(2 B u Fy Fz)] exp[ -1/2 (y / Fy)2] x {exp[-1/2[(z + H) / Fz)2] +
exp [ -1/2 [(z – H)/Fz]]2}
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2.8. Dispersion Modeling
contd…
Example
1.
A source emits 0.01 kg/sec of a Sox on a sunny summer afternoon with an
average wind speed of 4 m/sec. The effective stack height has been
determined to be 20 m. Find the ground level concentration 200 m
downwind from the stack.
A sunny afternoon would give
curve B
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2.8. Dispersion Modeling
contd…
Now from the figures:
Fy = 36 m
Fz = 20 m
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2.8. Dispersion Modeling
contd…
Now x = 200 m, y = 0, z = 0 m, and:
C(x,y,z) = [Q/(2 B u Fy Fz)] exp[ -1/2 (y / Fy)2] x {exp[-1/2[(z + H) / Fz)2] +
exp [ -1/2 [(z – H)/Fz]]2}
C(200,0,0) = [0.01 / (2 x 3.14 x 4 x 36 x 20)] x { exp[ –1/2(0/36)] x
{exp[ -1/2 x [(0 – 20)/20]2] = exp[ -1/2 x [(0 + 20)/20]2]}}
= 6.7 x 10-7 kg/m3 = 670 g/m3
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2.8. Dispersion Modeling
contd…
Special Conditions
If the measurement is taken at ground level and the plume is
emitted at ground level (Z = 0, H = 0):
C(x,y,z) = [Q/(2 B u Fy Fz)] exp[ -1/2 (y / Fy)2 ]
If the emission is at ground level and the pollutant is measured at
ground level on the center line in the direction of the wind (H = 0,
z = 0, and y = 0), the equation is even further simplified to:
C(x,y,z) = [Q/(2 B u Fy Fz)]
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2.8. Dispersion Modeling
contd…
3. Lagrangian model — a Lagrangian dispersion model
mathematically follows pollution plume parcels (also called
particles) as the parcels move in the atmosphere and they
model the motion of the parcels as a random walk process.
• The Lagrangian model then calculates the air pollution
dispersion by computing the statistics of the trajectories of
a large number of the pollution plume parcels.
• A Lagrangian model uses a moving frame of reference as
the parcels move from their initial location.
• It is said that an observer of a Lagrangian model follows
along with the plume.
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2.8. Dispersion Modeling
contd…
4. Eulerian model — an Eulerian dispersions model is
similar to a Lagrangian model in that it also tracks the
movement of a large number of pollution plume
parcels as they move from their initial location.
• The most important difference between the two
models is that the Eulerian model uses a fixed threedimensional Cartesian grid as a frame of reference
rather than a moving frame of reference.
• It is said that an observer of an Eulerian model
watches the plume go by
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2.9. Applications of Dispersion Modelling
(Ref: http://www.mfe.govt.nz/publications/air/good-practice-guide-atmospheric-dispersion-modelling/3-specialised-applications)
1.
2.
3.
4.
5.
Airshed modelling
Roadway emissions modelling
Modelling coastal fumigation
Visibility modelling
Dispersion modelling on larger scales
1. The regional scale
2. Long-range transport
6. Salt and steam effects: cooling towers
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2.9. Applications of Dispersion Modelling
contd…
The effect of thermal and
mechanical turbulence combining
to produce a well-mixed zone of
contaminants
Coastal fumigation
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2 marks
1.What is adiabatic lapse rate?
The decrease of atmosphere temperature with
height.
2. List out any four sampling methods.
Sedimentation, filtration, Impingement, ESP
3.National Ambient Air Quality Standard for ozone
(NAAQS)
Primary standard to protect public health. One-hour
average ozone > 0.12 ppm for federal standard. Onehour average ozone > 0.09 ppm for state standard.
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2 marks
contd…
4.Define Ambient air quality
• A physical and chemical measure of the concentration
of contaminants in the ambient atmosphere. The
quality is usually monitored over a specific period.
5.What is the word MINAS stands for?
• Minimum National Air Quality Standards
6.What is Mixing Height
• Height above the earth’s surface to which related
pollutants will extend, primarily through the action of
atmospheric turbulence
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2 marks
contd…
7.Define Fumigation
The phenomenon in which pollutants that are aloft
in the air are brought rapidly to ground level when
the air destabilizes.
8.Define Dispersion.
The mixing of gases contain the high concentration of
pollutant.
9.Write short note on Air monitoring.
The process of detention and measurement of
pollutants in air.
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2 marks
contd…
10. Define Mass concentration
• Concentration expressed in terms of mass of a substance
per unit volume of gas or liquid.
11. What do you meant by Pressure drop
• The differential pressure b/w two points in a system. The
resistance to flow b/w in the two points.
12. Relative humidity
• The ratio of the actual vapors pressure of the air to the
saturation vapor pressure.
13. What is Inversion
• Condition in the atmosphere in which air temperature
increases with elevation, under this conditions, the
atmosphere is said to be in stable equilibrium.
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2 marks
contd…
14. What is Lofting?
• A type of plume which occurs when an inversion exits
only below the plume and the plume is inhibited from
mixing downward.
15. What is Looping?
• A type of plume which has a wavy character. It occurs
in a highly unstable atmosphere because of rapid
mixing.
16.What is Chimney?
• A structure with an opening or outlet from or through
which any air pollutant may be emitted.
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2 marks
contd…
17.Define Coning
• A type of plume which is like a cone. This take place in a near
neutral atmosphere when the wind velocity is greater than 32km/h.
18. What Is Plume
• The path and extent in the atmosphere of the gaseous effluent
released from the source,usually a stack.
19.Briefly explain Chimney effect:
• The vertical penetration of smog through the inversion layer on the
south slope of the San Gabriel and San Bernardino Mountains
caused by the strong solar heating in the afternoon.
20.Define Fall out
• A radioactive pollutant in the air caused after the explosion of a
nuclear device, its degree of
• contamination depending on several factors, such as distance, wind,
and power of the device.
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